The present invention relates generally to explosive device and, in particular, to a stemming plug to be used in a borehole.
Stemming materials and plugs for use in boreholes or blast holes are known to the art. Stemming is the process in which material is placed into a bored blast hole on top of the explosive charge to contain or confine the explosive energy. Stemming the borehole keeps the blast energy from escaping out of the hole and concentrates the explosive energy within the blast hole. For example, if the user is blasting in rock, stemming improves fragmentation of the solid rock around the blast and increases the production of crushed rock. Further, stemming material can prevent contamination of the charge in the blast hole and also reduces the amount of ejected material.
Traditionally, blasters have used the indigenous material to stem the borehole. For example, in rock quarries or other accessible locations, crushed rock is the preferred stemming material. Coal mines use drill cuttings as stemming since crushed rock is not available. However, in wet conditions, drill cuttings are unsatisfactory because they offer little confinement of the blast, blowing out of the borehole in a stream of mud.
Mechanical stemming plugs also are used to confine the blast in the borehole. Tamping of boreholes to contain a explosive charge was taught almost 150 years ago in the "Rudimentary Treatise on the Blasting and Quarrying of Stone" by Maj.-Gen. Sir John Burgoyne, London, 1849. Gen. Burgoyne taught the use of iron tamping plugs in the shape of a barrel, a cone and a cone with wedges. Generally, then as now, the stemming plug is placed above the charge to enhance and hold the stemming material such as crushed rock. Mechanical stemming using a conical stem is shown in U.S. Pat. No. 4,754,705, to Worsey. When use correctly, stemming plugs can reduce flyrock, improve fragmentation, reduce airblast, expand borehole patterns and increase crushing. Such improvements allow the use of less explosive powder per ton of finished product.
The prior art stemming plugs, from Burgoyne to Worsey have similar failings. The substantially rigid, formed plugs normally do not fit snugly in the borehole and frequently require the use of alignment tools that do not always properly aligned the plug in the borehole. Air decking applications are not feasible with these rigid stemming plugs due to variations in borehole sizes.
Borehole size variations are also possible with gradual wearing of the drill. It is common for hole sizes to be undersized as much as 1/4 inch in diameter. Some stemming plugs function well in a smooth and consistent drill hole diameter, but collapse as the outer circumference of the plug proves too large for the reduced hole circumference. A stemming plug is needed to eliminate concern over borehole diameter variations and any irregular borehole surface, such as a protruding rock that can obstruct the rigid stemming plugs during insertion or which may collapse other, more flexible designs.
It would, therefore, be advantageous to have a stemming plug that is easily seated in the borehole, that does not require the use of an alignment tool, that can conform to varied circumferences and shapes of the borehole, can minimize contamination of the borehole and function in air decking and pre-split applications.